Conventionally, a gas sensor has been known which detects concentrations of predetermined gas components, such as NOx and oxygen, in a gas under measurement, such as an exhaust gas from a vehicle. Such a gas sensor includes a connector for electrical connection between an external device and a sensor element that detects a gas concentration. The connector is electrically connected to a plurality of front-surface electrodes arranged side by side on a front surface of the sensor element and to a plurality of back-surface electrodes arranged side by side on a back surface of the sensor element. For example, Patent Document 1 describes a gas sensor including a planar sensor element that detects a gas concentration, lead wires, and a connector that connects the sensor element and the lead wires. FIG. 27 and FIG. 28 are an exploded perspective view and a front view, respectively, illustrating a connector 300 of such related art. As illustrated, the connector includes a plurality of long narrow contact fittings connected to lead wires 316, two housings 302a and 302b configured to hold the contact fittings, a securing fitting 304 configured to secure the housings 302a and 302b, two U-shaped pressure springs 306a and 306b attached to the securing fitting 304, and an annular metal tube 308. In the connector 300, the contact fittings 310 and a sensor element 312 are clamped between the housings 302a and 302b, which are secured by the securing fitting 304. Thus, electrodes 314 of the sensor element 312 are in contact with and are electrically connected to the contact fittings 310. Crimping the outer periphery of the metal tube 303 causes the inner periphery of the metal tube 308 to displace the pressure springs 306a and 306b. With the pressing force of the pressure springs 306a and 306b, the contact fittings 310 and the electrodes 314 are pressed under a predetermined pressure. This prevents the occurrence of defective contact between the contact fittings 310 and the electrodes 314 caused by vibration.    Patent Document 1: Japanese Examined Utility Model Registration Application Publication No. 6-37326 (FIG. 1(b))